The B-17 Flying Fortress was the first large, four-engined heavy bomber and is still one of the most recognized airplanes ever built. It was most widely used for daylight strategic bombings of German industrial targets during World War II as part of the US Eighth Air Force.

B-17 "Sally B", England, 2001.

The prototype of the B-17 first flew on July 28, 1935. Few B-17s were in service when the United States entered World War II on December 7, 1941, but production quickly accelerated. The aircraft served in every WWII combat zone. Production ended in May 1945 after 12,726 aircraft had been built.

Contrary to the public perception that the aircraft was named the "Flying Fortress" because of the number of heavy machine guns it carried, the B-17 in fact received that sobriquet from newspaper reporters in the 1930s based on its original mission as a coastal patrol bomber, a 'flying fortress' that would guard the nation's offshore limits beyond the range of the heavy guns sited at major harbors. Among the combat aircrews that flew bombers in World War II, noted aviation writer Martin Caidin reported that the B-17 was referred to as the "Queen of the Bombers."

The B-17 was noted for its ability to take battle damage, still reach its target and bring its crew home. It reportedly was much easier to fly than its contemporaries, and its toughness more than compensated for its shorter range and lighter bomb load when compared to the Consolidated B-24 "Liberator" or the British "Lancaster" heavy bombers.

The design went through eight major changes over the course of its production, culminating in what some consider the definitive type, the B-17G, differing from its immediate predecessor by the addition of a chin turret with two .50-caliber machine guns under the nose. This eliminated the airplane's main defensive weakness.

Prior to the introduction of the P-51 Mustang long range escort fighter, a B-17 escort variant called the YB-40 was introduced. This aircraft differed from the standard B-17 in that a second dorsal turret was installed between the top turret and the waist guns; and the single .50-caliber machine gun at each waist station was replaced by a pair of .50-calibers. In addition, the bombardier's equipment was removed and replaced with two .50-caliber machine guns to augment the existing 'cheek' machine guns, and the bomb bay itself was converted to a magazine. The concept was twofold. First, the YB-40 would provide a heavily-gunned escort capable of accompanying the bombers all the way to the target and back. Second, they were used as decoys; a YB-40 would leave the bomber stream with one engine feathered, apparently in distress. Enemy fighters would close for the kill and discover that the 'cripple' was nothing of the kind. The aircraft was used with some success in the Mediterranean, but as noted was rendered obsolete by the development of the "Mustang."

Late in WWII, at least 25 B-17s were loaded with 12,000 pounds (5.4 tonnes) of high explosives, fitted with radio controls, dubbed "BQ-7 Aphrodite missiles," and used against U-boat pens and bomb-resistant fortifications. Because few (if any) BQ-7s hit their target, the Aprodite project was scrapped in early 1945.